This disclosure generally relates to methods for fabricating articles; more particularly, this disclosure relates to methods for reducing surface roughness of articles, such as, but not limited to, metal articles formed by additive manufacturing processes.
Manufacturing methods that rely on the addition of material to “build” components portion-by-portion, such as layer-by-layer, often suffer from unduly high levels of surface roughness, attributable in part to incomplete leveling of surfaces formed, for example, by melted (or partially melted) and solidified powder feed-stocks. Spray-forming and thermal spraying are two such processes used to form coatings or freestanding articles. The so-called “additive manufacturing” methods are further examples of such processes, and these methods are of particular interest to industry for their potential to fabricate complex three-dimensional parts with reduced cost and increased throughput relative to conventional metalworking processes such as casting and forging. The term “additive manufacturing” is defined by the American Society for Testing and Materials as the “process of joining materials to make objects from three-dimensional model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining and casting.” Such processes have demonstrated capability to manufacture components with complex features, including, for example, internal channels for facilitating fluid flow, such as for cooling or fluid delivery.
High surface roughness on external surfaces or internal channel walls may act to hinder component functionality where, for example, fluid flow plays a role in the working of the component. For example, turbine airfoil components such as blades and vanes typically specify upper limits for roughness of certain external surfaces to maintain aerodynamics of gas flow within design parameters. Moreover, components that facilitate flow of liquid are typically desired to have flow channels, such as internal flow channels, with channel wall surface roughness below specified limits to promote efficient flow and reduce fouling of channels by debris. Finally, unduly high surface roughness may also detract from mechanical properties of articles; for instance, high surface roughness may promote fatigue crack initiation in some applications, reducing the life of components relative to those having a smoother surface.
Given the potentially detrimental effects of high surface roughness, there is a need for methods to reduce surface roughness for components, such as components fabricated by additive manufacturing methods, where surface roughness issues are common.